Microsoft's first trackball, the IntelliMouse Trackball, was not a big hit. It did not fit the hand as well as a Logitech, and it didn't have their optical tracking system. Now Microsoft is going back to balls with the Trackball Explorer and Trackball Optical, their naming coinciding with the names for Microsoft's mice. The most important difference between the Trackball Explorer and Optical is that the Optical is designed so that you move the ball with just your thumb, and there really is no way around that. The Explorer has the ball mounted in the center so your middle three fingers can keep it rolling continuously (important when gaming).